Ultra high strength twip steel sheet and manufacturing method thereof

ABSTRACT

Disclosed herein is an ultra high strength TWIP steel sheet having an austenite matrix texture, comprising 0.15˜0.30 wt % of carbon, 0.01˜0.03 wt % of silicon, 15˜25 wt % of manganese, 1.2˜3.0 wt % of aluminum, 0.020 wt % or less of phosphorus, 0.001˜0.002 wt % of sulfur, and residual iron and other inevitable impurities, wherein the TWIP steel sheet is cold-rolled at a reduction ratio of 35˜40% per pass under a condition of 180˜220 Mpa in front and rear tension, and thus has an average plastic strain ratio of 1.2 or more. The ultra high strength TWIP steel sheet has an improved average plastic strain ratio and excellent formability because it has a developed Goss orientation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims under 35 U.S.C. §119(a) priority to Korean Application No. 10-2008-0070011, filed on Jul. 18, 2008, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a twinning induced plasticity (TWIP) steel sheet in which both slip and twin serve as a deformation mechanism at the time of plastic deformation, and a method of manufacturing the same. More particularly, the invention relates to a twinning induced plasticity (TWIP) steel sheet for vehicle body components, which has a high plastic strain ratio (r value), and a method of manufacturing the same.

2. Background Art

Generally, ultra high strength steel sheets, which are widely used as materials for automotive body components, have a tensile strength of 590˜780 MPa, a yield strength of 270˜350 MPa, an elongation rate of 25˜35% and a plastic strain ratio of 0.9˜1.2.

When applying these ultra high strength steel sheets to automotive body components, cracks, corrugating, and the like, can be caused by an insufficient elongation rate at the time of press forming. Thus a thick steel sheet is being used considering the strength of vehicle body components. Further, even though elongation is thus ensured, it can then be difficult to form a steel sheet into vehicle body components because the vehicle body components are complicated and multi-functionalized. Therefore, there is a need in the art to increase the plastic strain ratio of a steel sheet by developing forming technologies.

Korean Unexamined Patent Application Publication No. 2007-0018416 (not published yet), incorporated by reference in its entirety herein, is directed to an ultra high strength TWIP steel sheet, comprising: 0.15˜0.30 wt % of carbon, 0.01˜0.03 wt % of silicon, 15˜25 wt % of manganese, 1.2˜3.0 wt % of aluminum, 0.020 wt % or less of phosphorus, 0.001˜0.002 wt % of sulfur, and residual iron and other inevitable impurities.

The demand for complicated vehicle body components is increasing, and, accordingly, further improvements in the plastic strain ratio of a steel sheet are required, the plastic strain ratio being an important factor influencing molding capability.

The above information disclosed in this the Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.

SUMMARY OF THE INVENTION

In one aspect, the present invention is directed to an ultra high strength TWIP steel sheet having improved plastic strain ratio, and a method of manufacturing the same.

In a preferred embodiment, the present invention provides a technology of remarkably improving the plastic strain ratio of a TWIP steel sheet under given conditions by suitably controlling textures, that is preferred orientations, during cold rolling rather than controlling composition of the TWIP steel sheet as disclosed in Korean Unexamined Patent Application Publication No. 2007-0018416, incorporated by reference in its entirety herein.

In particularly preferred embodiments, an ultra high strength TWIP steel sheet according to the present invention includes 0.15˜0.30 wt % of carbon, 0.01˜0.03 wt % of silicon, 15˜25 wt % of manganese, 1.2˜3.0 wt % of aluminum, 0.020 wt % or less of phosphorus, 0.001˜0.002 wt % of sulfur, residual iron and other inevitable impurities. In further preferred embodiments, the ultra high strength TWIP steel sheet is suitably cold-rolled at a reduction ratio of 35˜40% per pass under the condition of 180˜220 Mpa of front and rear tension, so that it has an average plastic strain ratio of 1.2 or more and has an Goss orientation as a main texture component.

According to preferred embodiments of the invention, most of the materials obtained from nature or from working are polycrystalline materials in the form of crystal aggregates. Accordingly, their crystallographic orientations are not random, and appear in specific orientations. In further preferred embodiments, these materials having ordered crystal orientations are referred to as materials having preferred orientations, that is, textures.

According to preferred embodiments, an austenite matrix metal plate, for example a TWIP steel sheet, preferably has a crystallographic texture including, but not limited to, copper orientation, Goss orientation, brass orientation, S orientation and cube orientation. Preferably, the relative volume fractions of these orientations influence the average plastic strain ratio of the austenite matrix metal plate.

According to other preferred embodiments, the crystal orientation of the metal plate produced through rolling is suitably defined by a rolling plane and a rolling direction. That is, the texture of the metal plate may be defined by a crystal plane placed suitably parallel to the rolling plane and a crystal direction placed suitably parallel to the rolling direction. Accordingly, in further embodiments, the crystal plane is represented by Miller indices {hkl}, and the crystal direction is represented by <uvw>. For example, in certain exemplary embodiments, copper orientation is represented by {112}<111>, Goss orientation is represented by {011}<100>, brass orientation is represented by {112}<110>, S orientation is represented by {123}<634>, and cube orientation is represented by {001}<100>.

According to other preferred embodiments of the invention, in a metal sheet having low stacking fault energy (SFE) such as a TWIP steel sheet, brass orientation is preferably particularly developed as a main orientation during cold rolling. Preferably, since its plastic strain ratio at an angle of 90° with respect to a rolling direction is low, its average plastic strain ratio thereof is also low, and thus there is a problem of rupturing or cracking at the time of forming. Accordingly, in the ultra high-strength TWIP steel sheet of the present invention, in addition to a brass-oriented texture, since a Goss-oriented texture in which a plastic strain ratio at an angle of 90° with respect to a rolling direction is improved is also developed, it has an average plastic strain ratio of 1.2 or more, preferably 1.5 or more, and its press formability is suitably improved.

In other further embodiments, a method of manufacturing the ultra high strength TWIP steel sheet according to the present invention preferably includes: cold-rolling of a hot-rolled steel sheet having an austenite matrix having the above composition at a reduction ratio of 35˜40% per pass under the condition of 180˜220 Mpa in front and rear tension; and annealing of the cold-rolled steel sheet at a temperature of 850˜900° C. The hot-rolled steel sheet can be suitably obtained by hot-rolling of a continuously-cast slab at a temperature of 1300˜1100° C. and then gradually cooling the hot-rolled slab at a cooling rate of 60° C./sec or less such that a martensite phase is not formed.

According to certain other embodiments, the cold-rolling of a TWIP steel sheet was performed during 5˜7 passes at a reduction ratio of 20˜30% per pass while suitably applying a front and rear tension of about 120 MPa thereto. In further embodiments, the front and rear tension was less than 120 MPa. Accordingly, with the increase of the front and rear tension, rolling reduction is suitably decreased, so that it is difficult to adjust a reduction ratio per pass to 30%, thereby increasing a process time, and that, according to other further embodiments, with the increase of the front and rear tension, it is difficult to control a texture after annealing. In further embodiments, it is commonly accepted that since the TWIP steel sheet, which is an ultrahigh tension steel sheet, has high strength, it is difficult to cold-roll the TWIP steel at a high reduction ratio. However, in the present invention, when a front and rear tension of about 120 MPa is applied, since the stress caused by rolling is suitably reduced, it is possible to cold-roll the TWIP steel at a high reduction ratio of about 40% per pass. Further, the number of passes can be suitably decreased.

It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum).

As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered.

The above features and advantages of the present invention will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated in and form a part of this specification, and the following Detailed Description, which together serve to explain by way of example the principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a graph showing the orientation distribution function (ODF) of a TWIP steel sheet according to a Comparative Example;

FIG. 2 is a graph showing the plastic strain ratio (R) to an angle relative to the rolling direction of a TWIP steel sheet according to the Comparative Example shown in FIG. 1;

FIG. 3 is a graph showing the orientation distribution function (ODF) of a TWIP steel sheet according to an Example; and

FIG. 4 is a graph showing the plastic strain ratio (R) to an angle relative to the rolling direction of a TWIP steel sheet according to the Example shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described herein, the present invention includes an ultra high strength TWIP steel sheet having an austenite matrix, comprising 0.15˜0.30 wt % of carbon, 0.01˜0.03 wt % of silicon, 15˜25 wt % of manganese, 1.2˜3.0 wt % of aluminum, 0.020 wt % or less of phosphorus, 0.001˜0.002 wt % of sulfur, and residual iron and other inevitable impurities, wherein the TWIP steel sheet is cold-rolled at a reduction ratio of 35˜40% per pass under a condition of 180˜220 Mpa in front and rear tension.

In one embodiment, the sheet has an average plastic strain ratio of 1.2 or more.

The invention also features a method of manufacturing an ultra high strength TWIP steel sheet having an improved average plastic strain ratio, comprising cold-rolling a hot-rolled steel sheet having an austenite matrix and comprising 15˜25 wt % of manganese at a reduction ratio of 35˜40% per pass under a condition of 180˜220 Mpa in front and rear tension, and annealing the cold-rolled steel sheet.

In one embodiment, the annealing is carried out at a temperature of 850˜900° C.

The invention also includes a motor vehicle comprising an ultra high strength TWIP steel sheet having an austenite matrix,

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings.

The ultra high strength TWIP steel sheet according to preferred embodiments of the present invention is a high manganese steel sheet including 15˜25 wt % of manganese. In particular embodiments, the ultra high strength TWIP steel sheet according to the present invention may have the same composition as that of the TWIP steel sheet disclosed in Korean Unexamined Patent Application Publication No. 2007-0018416 (refer to Table 1), incorporated by reference herein. The reason why the amounts of the components included in the TWIP steel sheet are limited is similar to that described in the specification of Korean Unexamined Patent Application Publication No. 2007-0018416.

TABLE 1 Component C Si Mn Al P S Fe Amount 0.15~0.30 0.01~0.03 15.0~25.0 1.20~3.00 0.020 0.001~0.002 Residue (wt %) or less

According to certain embodiments of the invention, in order to evaluate the material properties of the ultra high strength TWIP steel sheet according to the present invention, a plurality of TWIP steel sheets having a composition shown in Table 2 below was fabricated, and in further preferred embodiments, tests for measuring the average plastic strain ratio thereof were suitably conducted.

TABLE 2 Component C Si Mn Al P S Fe Amount (wt %) 0.22 0.03 21 2.0 0.01 0.001 Residue

According to further preferred embodiments, the TWIP steel sheet used in the test was suitably manufactured as follows. First, the composition shown in Table 2 was melted in an electric furnace and then continuously cast to suitably obtain a slab, and then the obtained slab was hot-rolled from 1300° C. to 1100° C. In further embodiments, the hot-rolled slab was gradually cooled to a temperature of 900˜600° C. at a cooling rate of 40° C./sec and then coiled to obtain a hot-rolled coil. In further exemplary embodiments, the obtained hot-rolled coil was cold-rolled, and then annealed at a temperature of 850° C. for 10 hours. The TWIP steel sheet according to preferred embodiments of the present invention has a single phase matrix, preferably mostly including austenite although very partially including martensite or ferrite.

According to other further embodiments, Table 3 shows the specific cold-rolling conditions and average plastic strain ratios of the TWIP steel sheets manufactured in Examples 1 to 6 and Comparative Examples 1 to 15. In certain embodiments, for example in the case of Examples 1 to 6, the cold-rolling conditions are preferably that front and rear tension is 180˜220 Mpa, and reduction ratio per pass (7 passes total) was preferably 30˜40%. In certain embodiments, for example in the case of Comparative Examples 1 to 15, the cold-rolling conditions were that front and rear tension is preferably 120˜230 Mpa, and reduction ratio per pass (7 passes total) was 20˜45%. According to preferred embodiments, each of the average plastic strain ratios was obtained by calculating the plastic strain ratios suitably measured at an angle of 0°, 45° and 90° with respect to a rolling direction.

TABLE 3 Tensile Annealing strength Reduction temp. Annealing Average Class. (MPa) ratio (%) (° C.) time R value Example 1 180 35 850 10 1.521 Example 2 180 40 850 10 1.611 Example 3 200 35 850 10 1.64 Example 4 200 40 850 10 1.66 Example 5 220 35 850 10 1.542 Example 6 220 40 850 10 1.533 Comparative 120 30 850 10 0.83 Example 1 Comparative 120 35 850 10 0.81 Example 2 Comparative 120 40 850 10 cut, Example 3 cannot be measured Comparative 140 30 850 10 0.78 Example 4 Comparative 140 35 850 10 0.79 Example 5 Comparative 140 40 850 10 cut, Example 6 cannot be measured Comparative 170 20 850 10 0.61 Example 7 Comparative 170 30 850 10 0.68 Example 8 Comparative 170 35 850 10 0.71 Example 9 Comparative 170 40 850 10 cut, Example 10 cannot be measured Comparative 180 30 850 10 0.72 Example 11 Comparative 180 45 850 10 cut, Example 12 cannot be measured Comparative 220 45 850 10 cut, Example 13 cannot be measured Comparative 230 20 850 10 cut, Example 14 cannot be measured Comparative 230 30 850 10 cut, Example 15 cannot be measured

According to other further embodiments, and as shown in Table 3, the average plastic strain ratios of the TWIP steel sheets in Comparative Examples 1 to 15 were about 0.83, and none of them exceeded 1.0, and the TWIP steel sheets in Comparative Examples 1 to 15 were cut and deteriorated. In other embodiments, all of the average plastic strain ratios of the TWIP steel sheets in Examples 1 to 5 were 1.5 or more, and the TWIP steel sheets in Examples 1 to 5 exhibited excellent average plastic strain ratio.

From the results shown herein, it can be seen that the reduction ratio of the TWIP steel sheet can be suitably increased only under the front and rear tension within a suitably predetermined range at the time of cold rolling. Accordingly, it is suitably determined that this fact is related to the formation of twin. That is, according to certain embodiments, it is determined that, in the case of the TWIP steel sheet which is a metal sheet having low stacking fault energy (SFE), its deformation is suitably maximized in a predetermined stress range because twin serves as a deformation mechanism, but it is deformed only by slip because the formation of twin is inhibited outside the predetermined stress range, and thus it is limited to accommodate external deformation.

IN certain exemplary embodiments, the texture and plastic strain ratio of the TWIP steel sheet in Example 1 are compared with those in Comparative Example 1 with reference to FIGS. 1 to 4.

Accordingly, in the TWIP steel sheet of Comparative Example 1, brass orientation was suitably developed as main orientation (refer to the orientation distribution function (ODF) graph shown in FIG. 1), and the average plastic strain ratio thereof was about 0.83, and the plastic strain ratios thereof at an angle of 90° with respect to a rolling direction were 0.7 or less (refer to FIG. 2).

In other embodiments, for example in the TWIP steel sheet of Example 1, Goss orientation as well as brass orientation was considerably developed (refer to the orientation distribution function (ODF) graph shown n FIG. 3). As a result, in further embodiments, the plastic strain ratios thereof at an angle of 90° with respect to a rolling direction were considerably increased (refer to FIG. 4), and the average plastic strain ratio thereof was about 1.521, which was increased to 160% or more of that of the TWIP steel sheet of Comparative Example 1.

As described herein, the present invention provides an ultra high strength TWIP steel sheet having an excellent average plastic strain ratio due to the development of a Goss-oriented texture.

Further, the present invention preferably provides an ultra high strength TWIP steel sheet, which can be cold-rolled at a high reduction ratio while suitably decreasing the number of passes, and which can realize press formability by improving the average plastic strain ratio 1.2 times, preferably 1.5 times.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 

1. An ultra high strength TWIP steel sheet having an austenite matrix, comprising 0.15˜0.30 wt % of carbon, 0.01˜0.03 wt % of silicon, 15˜25 wt % of manganese, 1.2˜3.0 wt % of aluminum, 0.020 wt % or less of phosphorus, 0.001˜0.002 wt % of sulfur, and residual iron and other inevitable impurities, wherein the TWIP steel sheet is cold-rolled at a reduction ratio of 35˜40% per pass under a condition of 180˜220 Mpa in front and rear tension, and thus has an average plastic strain ratio of 1.2 or more.
 2. A method of manufacturing an ultra high strength TWIP steel sheet having an improved average plastic strain ratio, comprising: cold-rolling a hot-rolled steel sheet having an austenite matrix and comprising 15˜25 wt % of manganese at a reduction ratio of 35˜40% per pass under a condition of 180˜220 Mpa in front and rear tension; and annealing the cold-rolled steel sheet at a temperature of 850˜900° C.
 3. The method of manufacturing an ultra high strength TWIP steel sheet according to claim 2, wherein the hot-rolled steel sheet comprises 0.15˜0.30 wt % of carbon, 0.01˜0.03 wt % of silicon, 15˜25 wt % of manganese, 1.2˜3.0 wt % of aluminum, 0.020 wt % or less of phosphorus, 0.001˜0.002 wt % of sulfur, and residual iron and other inevitable impurities
 4. The method of manufacturing an ultra high strength TWIP steel sheet according to claim 2, wherein the hot-rolled steel sheet is obtained by hot-rolling a continuously-cast slab at a temperature of 1300˜1100° C. and then cooling the hot-rolled slab at a cooling rate of 60° C./sec or less.
 5. An ultra high strength TWIP steel sheet having an austenite matrix, comprising 0.15˜0.30 wt % of carbon, 0.01˜0.03 wt % of silicon, 15˜25 wt % of manganese, 1.2˜3.0 wt % of aluminum, 0.020 wt % or less of phosphorus, 0.001˜0.002 wt % of sulfur, and residual iron and other inevitable impurities, wherein the TWIP steel sheet is cold-rolled at a reduction ratio of 35˜40% per pass under a condition of 180˜220 Mpa in front and rear tension.
 6. The ultra high strength TWIP steel sheet of claim 5, wherein the sheet has an average plastic strain ratio of 1.2 or more.
 7. A method of manufacturing an ultra high strength TWIP steel sheet having an improved average plastic strain ratio, comprising: cold-rolling a hot-rolled steel sheet having an austenite matrix and comprising 15˜25 wt % of manganese at a reduction ratio of 35˜40% per pass under a condition of 180˜220 Mpa in front and rear tension; and annealing the cold-rolled steel sheet.
 8. The method of claim 7, wherein the annealing is carried out at a temperature of 850˜900° C. 